Effects of Disc Injuries on the Mechanical Properties of the Lumber Disc

Author: Tirad Sulaiman Alsharari

Alsharari, Tirad Sulaiman, 2018 Effects of Disc Injuries on the Mechanical Properties of the Lumber Disc, Flinders University, College of Science and Engineering

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Background: Intervertebral disc tears can cause spinal disorders such as chronic low-back pain (LBP) and are linked to increasing age and disc degeneration; 80% of the population suffer from them at some point in their lives (Andersson 1999; Buckwalter 1995; Adams & Roughley 2006). LBP is a condition frequently experienced throughout the course of many people’s lifetimes and has major financial effects on the individual and society (White 1990). Its cause is ascribed to work-related injuries, to strenuous daily routine activities and to leisure activities, such as sports. The disorder has become a widespread problem and influences the quality of life of affected individuals (Urban & Roberts 2003). There are three distinct types of annular disc tears: rim lesions, concentric (circumferential) tears and radial tears. Rim lesions are more pronounced at the human spinal level T12/L1. Radial tears and concentric tears occur in equal frequency at level L4/L5 of the spine (Hilton et al. 1976; Thompson et al. 2000).

There have been limited in vitro studies investigating the effects of disc injuries on the mechanical behaviour of functional spinal units (FSU) in certain degrees of freedom (DOF) (e.g. flexion–extension, bending, axial rotation and compression). Under physiological conditions, mechanical tests have been conducted on FSUs with acute injuries inflicted on their discs to simulate annular disc tears. The findings show consensus in some areas, such as on the effects of tears in terms of various DOF; however, they lack agreement in others. For example, while FSU stiffness was found to decrease for rim lesions in axial rotation, it seemed to remain unaffected, to increase or sometimes to decrease for radial tears in flexion–extension (de Visser et al. 2007; Michalek& Iatridis 2012; Thompson et al. 2004; Thompson et al. 2000). The effects of the injuries were measured under varying simulated physiological conditions (e.g. inclusive or exclusive of a compressive preload and a posterior element) and sometimes in the presence of other potentially affecting factors (e.g. varying levels of disc degeneration and/or in combination with other injuries), which may explain any inconsistency found in the results. To the author’s knowledge, no studies have examined the six degrees of freedom (6DOF) in terms of the functioning of FSUs with disc injuries.

Objectives: The primary aim of this thesis was to evaluate the effects of disc injuries, particularly rim lesions and radial tears, on the mechanical and viscoelastic properties of ovine lumbar FSUs. These properties were evaluated under 6DOF dynamic testing, mimicking three physiological disc conditions that occur during walking and through office work (Costi et al. 2008). This study also compared the failure peak loads of the specimens exhibiting the two types of injuries after being subjected to traumatic overload.

Methods: Sixteen healthy sheep L4/5 FSUs (vertebra–disc–vertebra with an intact posterior element) of ages ranging from one to three years were randomly assigned into two groups: (1) for radial tears and (2) for rim lesions. Before and after the creation of injuries, each FSU was immersed in a hydration bath overnight under a preload equivalent to its disc size; it was then subjected to 6DOF dynamic testing at 1 Hz. 0.1 Hz and 0.01 Hz. After this, traumatic overload was applied to achieve FSU failure. The stiffness (the force/translation or moment/rotation) and the phase angle (the temporal shift between loading and displacement) were calculated for each DOF and at each frequency using the collected data and considering the peak load at which each FSU failure occurred.

Results: Under 6DOF dynamic testing, radial tears and rim lesions significantly decreased FSU stiffness in axial rotation (mostly toward injury location) at 1 Hz and 0.1 Hz, and in left and right lateral shear at 0.1 Hz. Testing at the lower frequency 0.01 Hz did not seem to make variation in the effects of both injuries. However, FSU phase angle displayed significant variation by radial tears and no changes by rim lesions. The change in frequency appeared to significantly determine the directions affected by radial tears and the patterns of effect. This control was clear by radial tears when an increase in phase angle changed from leftright lateral shear at 0.1 Hz to flexion- extension at 0.01 Hz. Such change in frequency also showed a decrease in phase angle by radial tears in posterior-anterior shear. The ultimate failure loads between specimens of radial tears and rim lesions resulting from a traumatic failure were not significantly different.

Conclusion: These findings can be clinically beneficial for example to people diagnosed with radial tears or rim lesions in their intervertebral discs. It raises their awareness and better understanding on the effects of directions they need to avoid where FSU stiffness can be most affected. Fields such as tissue engineering can benefit from understating a viscoelastic property of disc under dynamic movement simulating that of the in-vivo condition.

Keywords: Lumbar, Disc, intervertebral, Spine, Biomechanics, Mechanics, Biomechanical, Mechanical, Ovine, viscoelastic, Viscoelasticity, properties, tears, tear, Injury, Injuries, property, properties, 6 DOF, Hexapod, Dynamic, Testing, radial tear, radial tears, rim lesion, rim lesions, concentric tear, concentric tears, circumferential tear,circumferential tears, herniation, Tirad, Sulaiman, Alsharari, John, Costi, Flinders, LBP, Lower Back Pain, axial rotation, lateral bending, lateral shear, anterior shear, Posterior shear, Compression, Sudden Overload, Failure, traumatic failure. Annular tears, Annular injuries, Disc Structural effects

Subject: Engineering thesis

Thesis type: Masters
Completed: 2018
School: College of Science and Engineering
Supervisor: John Costi